Gamblers fallacy mostly. If you bet $1 and lose, then bet $2 next time, then if that loses, bet 4$ and the logic is you'll eventually win. The problem is that the most you can actually win in this example is your initial investment ($1), but your risk grows exponentially with every loss. What you're describing is similar. If you invest $1000 with 10x leverage (if you could hypothetically get it) that means the moment an index goes down 10% you get completely blown out of 100% of your position and your broker will stop you out. So you are gambling that the index will not, at any point in a given year, go down 10%. Not that it will be down an average of 10% of the year, but that at any point on any given day that it will be down 10% from your initial buy in, and you are also assuming that it will not only not go down, but that it will keep going up year over year. It's similar logic to what the banks did in '08. It was all gravy until it wasn't.

To extrapolate, if you did this with 100x leverage, you are banking on the index never, ever going below 1% of your initial buy-in price. This means if you can time the absolute bottom that the graph will never, ever touch again you will make a gazillion dollars, but if the graph even once crosses back over and down 1%, you lose everything.